28
enhance the flux of fixed carbon to pyruvate, the rate of nitrogen assimilation was
slowed down by the moderate reduction of glutamine synthetase I through CRISPRi.
The resulting high CO 2 requiring phenotype mutant showed the highest L-lactate
isomer production rate (2.2 mM/day) obtained by a cyanobacterium with an accumulation of 9 mM of product in 4 days. Tong et al. (2011) employed an alternative
way to produce L-lactate from carbon dioxide and ethanol. In this process, the pyruvate produced by the pyruvate decarboxylase (PyDC) from carbon dioxide and acetaldehyde is subsequently reduced by the lactate dehydrogenase (LDH) in L-lactate.
The regeneration of the NADH cofactor is achieved afterwards by the oxidation of
ethanol in acetaldehyde performed by the alcohol dehydrogenase (ADH).
In Varman et al. (2013), the specific production of the D-lactate isomers from
Synechocystis resulted from the co-expression of a mutated glycerol dehydrogenase
(encoded by GlyDH) (Wang et al. 2011) and of a soluble NADPH/NADH transhydrogenase from Pseudomonas aeruginosa (encoded by Sth). The engineered cyanobacterium showed the final triter of 12.7 mM after 30 days under photoautotrophic
condition. In Li et al. (2015), a strain of Synechococcus elongatus PCC 7942 was
modified with the introduction in its neutral site I of an optimized D-lactate dehydrogenase (ldhD) gene from Lactobacillus bulgaricus ATCC11842 engineered to
use NADPH cofactor together with the lactate permease (lldP) gene from E. coli
MG1655, both under the control of an inducible IPTG promoter. The highest production of 14.54 mM of D-lactate was achieved after 10 days of culture under continuous light with 5% CO 2 (v/v). Different strategies to produce D-lactate isomers
were undertaken in Hirokawa et al. (2017a). The authors reasoned that a pathway
possessing a higher carbon flux than the ordinarily employed glycolysis could be
more profitably exploited to produce lactate. Since the Calvin–Benson cycle is perfectly positioned to fulfill this requirement, Hirokawa et al. (2017a) proposed to rely
on dihydroxyacetone phosphate (DHAP) originating from the Calvin–Benson cycle
to engineer a lactate-forming pathway. More precisely, the authors modified S. elongatus PCC 7942 by engineering a synthetic pathway consisting of methylglyoxal
synthase, which synthesizes methylglyoxal from DHAP, and glyoxalases I and II,
which transform methylglyoxal to lactate. A further increase in lactate production is
achievable by devising a cyanobacterium featuring improved lactate export capacity. Among the plausible approaches stands out engineering a lactate/H
+
symporter
or adopting a host where the native lactate export system is highly efficient.
1.5.3 Polyhydroxyalkanoates
Polyhydroxyalkanoates (PHAs) are biologically produced polyesters expected to
replace petroleum-based counterparts. PHAs are resistant to hydrolysis and UV
irradiation and insoluble in water though they are biodegradable and biocompatible
(Raza et al. 2018). These properties make PHAs a great precursor for the creation of
A. A. Azim et al.
enhance the flux of fixed carbon to pyruvate, the rate of nitrogen assimilation was
slowed down by the moderate reduction of glutamine synthetase I through CRISPRi.
The resulting high CO 2 requiring phenotype mutant showed the highest L-lactate
isomer production rate (2.2 mM/day) obtained by a cyanobacterium with an accumulation of 9 mM of product in 4 days. Tong et al. (2011) employed an alternative
way to produce L-lactate from carbon dioxide and ethanol. In this process, the pyruvate produced by the pyruvate decarboxylase (PyDC) from carbon dioxide and acetaldehyde is subsequently reduced by the lactate dehydrogenase (LDH) in L-lactate.
The regeneration of the NADH cofactor is achieved afterwards by the oxidation of
ethanol in acetaldehyde performed by the alcohol dehydrogenase (ADH).
In Varman et al. (2013), the specific production of the D-lactate isomers from
Synechocystis resulted from the co-expression of a mutated glycerol dehydrogenase
(encoded by GlyDH) (Wang et al. 2011) and of a soluble NADPH/NADH transhydrogenase from Pseudomonas aeruginosa (encoded by Sth). The engineered cyanobacterium showed the final triter of 12.7 mM after 30 days under photoautotrophic
condition. In Li et al. (2015), a strain of Synechococcus elongatus PCC 7942 was
modified with the introduction in its neutral site I of an optimized D-lactate dehydrogenase (ldhD) gene from Lactobacillus bulgaricus ATCC11842 engineered to
use NADPH cofactor together with the lactate permease (lldP) gene from E. coli
MG1655, both under the control of an inducible IPTG promoter. The highest production of 14.54 mM of D-lactate was achieved after 10 days of culture under continuous light with 5% CO 2 (v/v). Different strategies to produce D-lactate isomers
were undertaken in Hirokawa et al. (2017a). The authors reasoned that a pathway
possessing a higher carbon flux than the ordinarily employed glycolysis could be
more profitably exploited to produce lactate. Since the Calvin–Benson cycle is perfectly positioned to fulfill this requirement, Hirokawa et al. (2017a) proposed to rely
on dihydroxyacetone phosphate (DHAP) originating from the Calvin–Benson cycle
to engineer a lactate-forming pathway. More precisely, the authors modified S. elongatus PCC 7942 by engineering a synthetic pathway consisting of methylglyoxal
synthase, which synthesizes methylglyoxal from DHAP, and glyoxalases I and II,
which transform methylglyoxal to lactate. A further increase in lactate production is
achievable by devising a cyanobacterium featuring improved lactate export capacity. Among the plausible approaches stands out engineering a lactate/H
+
symporter
or adopting a host where the native lactate export system is highly efficient.
1.5.3 Polyhydroxyalkanoates
Polyhydroxyalkanoates (PHAs) are biologically produced polyesters expected to
replace petroleum-based counterparts. PHAs are resistant to hydrolysis and UV
irradiation and insoluble in water though they are biodegradable and biocompatible
(Raza et al. 2018). These properties make PHAs a great precursor for the creation of
A. A. Azim et al.